Starlight Reveals a Clue to Biomolecular Handedness

January 19, 2010

Starlight Reveals a Clue to Biomolecular Handedness

Nobody knows why life on Earth is built from left-handed amino acids. But an important clue lies in the light coming from the Orion Nebula, say astrobiologists.

One of the great mysteries of life on Earth is that it is built almost exclusively from one enantiomer: left-handed amino acids and right-handed sugars. The puzzle is that left and right-handed versions of molecules are produced in equal amounts in all but the most exceptional circumstances. So what conditions led to terrestrial life’s preference for one over the other?

Today, Tsubasa Fukue at the National Astronomical Observatory of Japan in Tokyo and a few mates say they think they know how so-called biomolecular homochirality may have come about. The evidence comes from their study of light from the Orion Nebula (above), a well known star nursery and one of the most spectacular objects in the sky.

The Orion Nebula is important because it contains regions that must be similar to the one in which the Sun formed some 5 billion years ago. “Studies of the Orion star-forming region enable us to investigate processes that may have occurred during the birth of our own solar system,” say Fukue and co.

Here’s the interesting discovery: Fukue and buddies say their study of the nebula reveals a huge area some 400 times the size of our Solar System that is bathed in circularly polarised visible light, probably generated by synchrotron radiation from particles accelerated in powerful magnetic fields.

That’s an important observation because on Earth, circularly polarised light can bias chemical reactions in a way that ensures that one enantiomer forms preferentially over another.

Could the same thing happen in space? Possibly. One problem is that amino acids require UV light for photolysis rather than visible light. But it’s simply not possibly to see UV light (polarised or not) from the Orion Nebula because it is absorbed by dust in the line of site between here and there.

Fukue and co say that various models of star formation predict that circularly polarised UV light ought to be produced in the region. We also know that amino acids are common in interstellar space.

So it’s quite possible that conditions are ripe in the Orion Nebula for homochirality. And if so, it is also possible that the Sun formed in a similar region of space and that the Earth was seeded with these molecules during a period of heavy bombardment soon after it formed.

That’s a neat idea but not an entirely new one. The debate over biomolecular homochirality and the role of circularly polarised light goes back many years and while Fukue and co add some interesting new evidence, it is by no means a slamdunk.

The best they can say is that their work supports the idea but it’s going to take something a little more substantial to settle the issue.

Ref: arxiv.org/abs/1001.2608: Extended High Circular Polarization in the Orion Massive Star Forming Region: Implications for the Origin of Homochirality in the Solar System